Extremely low lattice thermal conductivity in light-element solid materials

Lattice thermal conductivity (κl) is of great importance in basic sciences and in energy conversion applications. However, low-κl crystalline materials have only been obtained from heavy elements, which typically exhibit poor stability and possible toxicity. Thus, low-κl materials composed of light elements should be explored. Herein, light elements with hierarchical structures in a simple square-net lattice as well as a small discrepancy in atomic mass and radius exhibit low κl. The hierarchical structure exhibits various chemical bonds and asymmetric geometry of building units, resulting in flat phonon branches and strong phonon–phonon interactions similar to those observed in heavy-element materials. These phenomena generate a large phonon anharmonicity, which is the prerequisite for achieving extremely low κl. For example, KCu4Se3 exhibits an extremely low κl of 0.12 W/(m·K) at 573 K, which is lower than that of most heavy-element materials. These findings can reshape our fundamental understanding of thermal transport properties of materials and advance the design of low-κl solids comprising light elements.